(1) Field of the Invention
The present invention is directed to a membrane electrode assembly (MEA) for a fuel cell which exhibits enhanced reversal tolerance. In particular, the invention provides an MEA configuration in which a layer of iridium or an iridium compound, preferably metallic iridium or iridium oxide supported on TiO2, is provided on the anode to electrolyze available water and pass the majority of the current during a reversal of the fuel cell, thereby preventing damage to the MEA.
(2) Description of Related Art
“Reversal” of a fuel cell (in which the cell operates in a regenerative manner, consuming electricity and, for example, electrolyzing water to produce hydrogen and oxygen) can occur during normal fuel cell operation due to “fuel starvation” attributable to poor flow sharing, or to low fuel flow. Fuel starvation can also occur during a shutdown event when the anode is purged with air (to leave the stack in a safe situation or in preparation for freeze start) and load is drawn to additionally bleed down cell voltages. Between start-up and shutdown (if H2 is not purged from the anode) air will permeate into the anode and consume H2 due to small leaks and/or cross-over from the cathode. On the resulting start-up, the exchange of air on the anode with H2 and subsequent load will result in fuel starvation at the cell outlet.
In conventional fuel cells which utilize, for example a platinum based anode catalyst, without any provision for reversal tolerance, cell reversal can cause damage to the anode, due to carbon oxidation. Accordingly, it is known to include additional measures which provide the MEA with a degree of “reversal tolerance”, thereby avoiding damage to the MEA:
One known technique for achieving reversal tolerance, for example, is to provide a Pt/Ru alloy or admix over the entire surface of the anode structure. However, while this MEA configuration is easy to manufacture, it has a number of disadvantages which render it less than ideal. First, among these is that during operation of the fuel cell, the ruthenium which is applied to the anode can gradually migrate from the anode to the cathode, which over time can contribute substantially to a degradation of the fuel cell output voltage, as can be seen in FIG. 1. Moreover, this technique also requires an excessive amount of costly precious metal.